[clang] [llvm] [KeyInstr] Add docs (PR #137991)

[Jeremy Morse via llvm-commits]

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+# Key Instructions debug info in LLVM and Clang
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+Key Instructions is an LLVM feature that reduces the jumpiness of optimized code debug stepping by discriminating the significance of instrucions that make up source language statements. This document explains the feature and how it is implemented in [LLVM](#llvm) and [Clang](#clang-and-other-front-ends).
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jmorse wrote:

"distinguishing" instead of "discriminating" perhaps to make it less-word-soupy? Or "ranking the significance"?

https://github.com/llvm/llvm-project/pull/137991